1. Field of the Invention
This invention relates to a transfer-gate circuit used with a complementary MOSFET integrated circuit (CMOS-IC).
2. Background of the Invention
The power consumed by a CMOS-IC is less than that consumed by an integrated circuit of the MOSFET single channel type. CMOS-ICs are now used in various fields, for example, in logic circuits, memory circuits and microcomputers. A prior art transfer-gate circuit used in a CMOS-IC is illustrated in FIG. 1. The transfer-gate has two MOSFETs 1 and 2 which are connected in parallel and are respectively N-channel type and P-channel type MOSFETs. Each source electrode of MOSFETs 1 and 2 is connected to an input terminal 3. The drain electrodes of the MOSFETs 1 and 2 are connected to an output terminal 4. An input signal applied to the input terminal 3 is transmitted to the output terminal 4 when the MOSFETs 1 and 2 become conductive, i.e., are switched ON.
The MOSFETs 1 and 2 are controlled by control signals .phi. and .phi. applied to the gate electrodes thereof. In the case of N-channel MOSFET 1, the MOSFET will be conductive, i.e. ON, when control signal .phi. is at a high level, and non-conductive, i.e. OFF, when control signal .phi. is a low level. The P-channel MOSFET 2 operates complementarily. Therefore, the transfer-gate circuit comprising MOSFETs 1 and 2 is controlled by control signals .phi. and .phi.. Control signal .phi. is the inverted signal of the control signal .phi.. The input signal is transferred to the output when the control signal .phi. is at a high level, but is not transferred when the control signal .phi. is low level.
For example, when the power supply voltage is five volts, the threshold voltage V.sub.th of the P-channel MOSFET 2 is in general set up at -0.8 V and that of the N-channel MOSFET 1 is set up at +0.8 V. But this Vth will be changed by the following various effects. First, V.sub.th varies corresponding to the gate length. Generally, it is difficult to maintain a uniform gate length dimension in the manufacturing process, and the length of the gate may have a slightly different value in each lot or wafer. This is because it is difficult in the present process to hold all the conditions of the process constant for each lot or wafer. Second, V.sub.th shifts in response to ionic radiation, for example, gamma-rays and X-rays, emanating from external sources. These rays may be produced from the material constructing the device. The value of V.sub.th will be shifted in the minus direction semipermanently. Third, V.sub.th may be effected by hot electrons. Hot electrons have high energy and can be accumulated in the gate insulation layer. Hot electrons can be produced by impact ionization which occurs in the P-N junction of the drain region of the MOSFET. If the V.sub.th of N-channel MOSFET 1 was shifted to a minus voltage, for example to -0.1 volts from +0.8 volts, the MOSFET 1 will not turn OFF when the control signal .phi. drops to its low level, namely ground potential, and the input and output terminals become the same level. As a result, the transfer-gate will lose the capacity to transfer signals in response to the control signals .phi. and .phi..
Next the noise suppression margin in this circuit is only a fraction of V.sub.th so this circuit is sensitive to noise. For example, assuming that the control signal .phi. is 0 volts and the control signal .phi. is 5 volts the transfer-gate is non-conductive, but MOSFET 2 will conduct if a noise level lower than -.vertline.V.sub.th .vertline. appears in the control signal .phi.. Therefore, the transfer-gate will malfunction. Generally, the noise present in a power supply signal is in the range of .+-.10%, but may at times exceed that amount. In these cases, the transfer-gate circuit could operate by mistake. Furthermore, when V.sub.th is maintained at a high value in order to increase the noise suppression margin, the range of usable power supply, which is determined by .vertline.V.sub.th .vertline. and the breakdown voltage of the circuit, is reduced because the power supply must be more than V.sub.th. Accordingly, the relationship of V.sub.th and the power supply potential is an important factor in the design of the circuit.